This invention relates to emulsion fuels and their use in gas turbines. More particularly, but not exclusively, the invention relates to emulsion fuels based on viscous hydrocarbons derived from crude oil or on directly produced high viscosity grades and the use thereof in gas turbines.
Fossil fuel power stations can be considered in two categories: steam thermal power stations and gas turbine power stations. In the former, fuel is burnt to heat water in a boiler and produce steam which drives a turbine; the turbine then drives a generator which produces electricity. In some cases, the steam is then condensed and returned to the boiler for reuse (a so-called xe2x80x9cclosed cyclexe2x80x9d). The efficiency of the conversion of fuel to electricity in steam thermal power stations can be as low as about 30%.
In gas turbine power stations, a fuel, such as gas or oil, is burnt in the combustor of a gas turbine so that the resulting combustion gases drive the turbine which drives a generator. The efficiency of this process is relatively low (about 30%) and many gas turbine power stations include a further step in which the hot combustion gases are used to heat water to steam, which is then used to drive a further generator. This further step may be in a closed cycle. The second step increases the efficiency of gas turbine power stations to as much as over 50%. Such a system is known as a xe2x80x9ccombined cyclexe2x80x9d system.
Refinery bottoms are the residual materials left after atmospheric distillation, vacuum distillation and other processes have been carried out in refineries as part of the conversion of crude oil to useful products. Such refinery bottoms can be used by burning them in power stations. Unfortunately, these residual materials are difficult to handle because of their high viscosities and, if they are to be used as components of fuel oil, they are normally xe2x80x9ccut-backxe2x80x9d with more valuable products obtained in distillation of the crude oil. The quality of refinery bottoms depends to a large extent on the quality of the crude oil from which they are derived and the cost effectiveness of use of refinery bottoms as a fuel depends upon the proportion of more valuable components of crude oil distillation with which they have to be cut back, although there is little alternative to this use as the refinery bottoms must be disposed of.
In general, it is unavoidable that certain undesirable components of the crude oil are likely to be concentrated in the refinery bottoms. Such undesirable components include sulphur and heavy metals such as vanadium and nickel. These components can lead to the formation of undesirable combustion products such as sulphur oxides and vanadium oxides which can be found in the emissions from power stations. Generally, such emissions need to be cleaned before they can pass into the atmosphere. Such extra treatment can reduce the overall efficiency of converting the liquid fuel into energy.
Naturally, it would be highly desirable to be able to use refinery bottoms, effectively a waste product from the refinery processing of crude oil, to produce electricity with a high conversion factor.
Emulsion fuels containing large amounts of water are more readily handleable than refinery bottoms. In this connection, reference is made to GB-A-974,042 which relates to oil-in-water emulsion fuels comprising a petroleum oil of high viscosity, the oil comprising 60 vol. % of the emulsion such that the emulsion has a viscosity of less than 150 S.S.F. at 77xc2x0 F. (25xc2x0 C.). GB-A-974, 042 proposes use of such oil-in-water emulsions in conventional steam thermal burners.
Also in the prior art, DE-A-275419 proposes the use of water-in-oil emulsions having a water content of from 3 to 8% in a gas turbine. Such emulsions consist of droplets of liquid water in a continuous oil phase and, at least insofar as the use of refinery bottoms may be contemplated, water-in-oil emulsions manufactured from refinery bottoms as a base fuel will have significantly higher viscosities than oil-in-water emulsions made from such a base fuel. In fact, there will be little significant difference between the viscosity of the water-in-oil emulsion and the viscosity of the base fuel itself.
In a first aspect of the invention, there is provided a hydrocarbon oil-in-water emulsion for use as gas turbine fuel, which emulsion is an oil-in-deionized water emulsion containing less than 1 ppm of sodium ions.
According to a second aspect of the present invention, there is provided a method of driving a gas turbine which comprises combusting a pre-atomized hydrocarbon oil-in-water emulsion in the combustor of the turbine.